1. Field of the Invention
This invention pertains to a window for transmitting microwave energy from one waveguide into another waveguide. More particularly, it pertains to a window for transmitting microwave frequency energy from one rectangular waveguide into another rectangular waveguide. Still more particularly, it pertains to a substantially circular microwave window for transmitting such microwave energy, together with microwave impedance matching structure.
2. Description of Related Art
Extensive development of new microwave tubes in recent years has produced higher output power and wider bandwidth requirements which exceed the limited capability of microwave windows of conventional design. Recent emphasis on millimeterwave tubes has added another difficulty in scaling conventional windows to higher frequencies where their dimensions become extremely small.
A window is needed because it frequently is desired to seal one wave guide pneumatically from the other waveguide. For example, one waveguide may be inside a microwave tube. Windows such as beryllia or alumina windows typically may be used. Beryllia is usually preferred, but because of difficulties in sealing beryllia to the metal walls of the wave guides, and the relative ease of sealing alumina to the wave guide metal walls, one might choose a window of alumina. Because of the higher dielectric constant of alumina, for a given transmission characteristic the alumina window will be smaller than the beryllia window.
It is desirable to explain the use of the window of this invention in connection with the transfer of microwave energy in the TE.sub.10 mode wherein the electric field intensity distribution is essentially uniform across the short or "b" dimension of the rectangular waveguides and varies in sinusoidal function across the long or "a" dimension of the rectangular waveguides.
According to the prior art, one might use a rectangular window having the same dimensions as the waveguide and as thin as practicable. That is, thick enough to support the difference in gas pressures between the two waveguides and easily machineable. However, such a window has a very narrow transmission bandwidth compared to the transmission bandwidth of a rectangular waveguide in a TE.sub.10 mode. Note also that a conventional thin window may be too thin to accommodate a reasonable pressure differential. Consequently, to use the widest bandwidth characteristics of the rectangular waveguide, one could not use such a window.
One might use a window such as that described in U.S. Pat. No. 3,860,891, entitled, "Microwave Waveguide Window Having the Same Cutoff Frequency as Adjoining Waveguide Section for an Increased Bandwidth" which issued Jan. 14, 1975 to Yukio Hiramatsu, the inventor herein. Such windows are typically about one half-wave length in thickness at some frequency in the middle of the passband of the window and of the waveguides, and they have matching transformer stubs 15 and 16 as shown, for example, in FIG. 6 of the patent. Such windows are particularly good when made of beryllia. They are also operable with alumina. However, although, as shown in FIG. 9 of the patent, the window 13 may have a diameter substantially equal to the short or "b" dimension of the waveguide for a particular frequency, in practice that rarely occurs. To obtain the desired bandwidth, the diameter is reduced, and as the dielectric constant of the window increases, the diameter of the window must further be decreased. Because alumina has a dielectric constant higher than beryllia, the diameter for alumina must, in the embodiment of the patent, be even smaller than that for beryllia. Consequently, an embodiment such as, for example, that of FIG. 17 of the patent must then be used to channel microwave energy into the region of the window.
Other means of focussing microwave energy might also be used. Such focussing also changes the bandwidth of the structure.
The window of the patent also may be used, as shown for example in FIG. 23 of the patent, to connect ridged waveguides.